1. Field of the Invention
The present invention relates to a printing apparatus and a printing method. More particularly, the present invention relates to a laser printing apparatus and a printing method capable of compensating for influence due to light intensity differences occurring on the surface of a photosensitive drum.
2. Description of the Related Art
Due to the recent widespread use of computers, peripherals such as printers are increasing in use. Printers are input with text or graphics from computers and print the input information on sheets of paper. The printers may be classified into dot matrix printers, ink-jet printers, or laser printers. The dot matrix printers are low in price, but have the disadvantages of loud noise and poor print quality. The laser printers have good speed and print quality, but may have the disadvantage of high price. Therefore, in general, ink-jet printers have been mostly used by individuals.
However, recent developments in printer manufacturing technologies enable laser printers to be produced at a low price, and thus laser printers are gradually being more widely used. The laser printers print sheet by sheet using the electrophotographic method. The elements of laser printers can be mainly divided into a controller part and an engine part. The controller part interprets image data sent from a computer, stores the interpreted image data in a random access memory (RAM) of the printer itself, communicates with the engine part so that the engine part can prepare for print tasks, and sends the data stored in the RAM in a serial data format. The engine part includes an organic photoconductive drum, a laser scanning unit (LSU), a developer, a cleaning unit, a charging unit, a transfer unit, and a fuser unit. If data to be printed is received, the printer performs, for print tasks, the processes of cleaning, charging, laser scanning, writing, developing, transferring, and fusing.
The laser scanning unit forms a latent image on the photoconductive drum in the laser scanning stage. Specifically, if laser beams scan the photoconductive drum charged at the same voltage, photocurrents are generated on the scanned portions of the drum so that negative charges on the drum are eliminated. Accordingly, the drum has negatively charged portions and charge-free portions, and, through a next developing stage, negatively charged toner particles stick on the laser beam-scanned surface of the drum on which a latent image is formed, to thus form the characters and/or graphics.
FIG. 1 is a block diagram illustrating a structure of a conventional laser printing apparatus. Referring to FIG. 1, the printing apparatus includes an interface unit 10, an image processing unit 20, a Pulse Width Modulation (PWM) unit 30, an LSU interface unit 40, and an LSU 50. The interface unit 10, the image processing unit 20, the PWM unit 30, and the LSU interface unit 40 belong to the controller part, and the LSU 50 belongs to the engine part.
The interface unit 10 receives image data to be printed from an externally connected user terminal. The received image data is generally input in a binary data format.
The image processing unit 20 generates and outputs information on individual print spots, that is, dot sizes to be output on a sheet of paper based on the received image data. That is, a number of dots are gathered together to produce one image, and dot sizes must be properly adjusted at every print spot in order to output a clear image. Thus, the image processing unit 20 generates the information on the sizes of individual dots so as to properly adjust dot sizes. In general, the information on the dot sizes is divided into levels ranging from 0 (all white) to 255 (all black).
The PWM unit 30 generates and outputs a pulse signal having a different pulse width during every print period based on the information on the dot sizes. That is, if the dot size has the highest level of 255, the PWM unit 30 outputs a high pulse during one period, and, if the dot size has the middle level, outputs a high pulse during half of a period.
The LSU interface unit 40 communicates between the controller part and the LSU 50, and produces a control signal corresponding to a pulse signal generated from the PWM unit 30 to enable the LSU 50 to output proper laser beams. The laser beams of the LSU 50 scan the photoconductive drum according to the control signal to form a latent image.
The photoconductive drum is a core part of the laser printer, and is formed with a cylindrical aluminum tube on the surface of which organic photoconductive material is coated. If a latent image is formed on the photoconductive drum by the LSU 50, toner powder is stuck on latent image-forming portions so as to be transferred on a sheet of paper.
FIGS. 2A-2D are graphs illustrating data output from individual components in the printing apparatus of FIG. 1. FIG. 2(A) illustrates a periodic pulse signal, in which each period indicates a period during which one dot is printed.
FIG. 2(B) illustrates input image data which is received from the interface unit 10, in which the input image data is binary data having 1's and 0's.
FIG. 2(C) illustrates information on the sizes of dots calculated in the image processing unit 20. The size of a dot to be printed during every period shown in FIG. 2(A) is sequentially calculated and sent to the PWM unit 30.
FIG. 2(D) illustrates a pulse signal having pulse widths each adjusted according to the dot size information. For “255”, the pulse value is maintained high during one entire period, for “128” (half of the above value), the pulse value is maintained high during half of a period, and for “64”, the pulse value is maintained high during a quarter of a period.
If the pulse signal is converted to a control signal through the LSU interface unit 40, the LSU 50 accordingly outputs laser beams. The laser beams form a latent image on the surface of the photoconductive drum as described above.
The photoconductive drum is manufactured in a cylindrical shape, and the LSU 50 is spaced a certain distance from the center of the round surface of the cylinder, so that it is inevitable that the laser beams from the LSU 50 reach the center of the surface of the drum and the sides of the surface of the drum in different light intensities.
FIG. 3 is a graph illustrating such differences in light intensity. Referring to FIG. 3, light intensity of laser beams on the center of the drum surface has a maximum value of about 0.3 mW, and the light beams gradually decrease in intensity as the beams move toward the left and right sides of the surface. For example, the light intensity drops down to about 0.27 mW on the portions of the surface about 10 cm away from the center.
With the light intensity differences occurring, latent images may not be properly formed on the portions spaced from the center of the surface of the photoconductive drum, which can cause obscure writing on the left and right portions of the surface as compared to the central portion. Therefore, the image quality can be degraded.